1. Technical Field
The present disclosure relates to a switched-capacitor band-pass filter of a discrete-time type, in particular for cancelling offset and low-frequency noise of switched-capacitor stages. For example, the present band-pass filter can be used in the control loop for driving capacitive gyroscopes made using MEMS technology.
2. Description of the Related Art
As is known, the use of micro-electro-mechanical systems (MEMS) has become increasingly widespread in various sectors of technology and has yielded encouraging results especially in the construction of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon micro-electro-mechanical structures comprising at least one mobile mass connected to a fixed body (stator) through springs and mobile with respect to the stator according to preset degrees of freedom. The mobile mass is moreover coupled to the fixed body via capacitive structures (capacitors). The movement of the mobile mass with respect to the fixed body, for example on account of an external stress, modifies the capacitance of the capacitors; the variation of capacitance can be exploited to detect the relative displacement of the mobile mass with respect to the fixed body and thus the applied force. Vice versa, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the mobile mass to set it in motion. In addition, to produce electromechanical oscillators, the frequency response of the inertial MEMS structures is exploited, which is typically of a second-order low-pass type.
Many MEMS (in particular, all the electromechanical oscillators and gyroscopes) envisage a driving device, which has the task of maintaining the mobile mass in oscillation.
Consequently, a driving system is provided, which controls in a precise way the movement of the mobile mass and includes a sensing amplifier operating in discrete-time mode. In order for the driving system to operate correctly with the desired precision level, it is useful to eliminate the offset of the sensing amplifier, as well as the so-called “flicker noise” or “1/f noise”, at low frequency, due, as is known, to random capture and release of charge carriers. In order not to interfere in the driving loop, filtering of the noise should not introduce phase shifts in the signal.
The same desires are also shared by other types of circuits, which would benefit from a discrete-time band-pass filtering, without the introduction of any phase shift.
In order to eliminate the offset of a micromechanical structure, derivative filters are normally used in the case of a continuous-time read chain; alternatively, a factory calibration is exploited. These solutions, in the case of drifts in the self-oscillation frequency of the mechanics due to ageing or temperature, do not enable a constant phase shift to be maintained, since the position of singularities, and thus the phase shift introduced at the frequency of interest, depends upon these parameters.
In literature, systems have been proposed using chopping techniques, which include a high-frequency offset modulation, filtering, and demodulation. These solutions require, however, a complex signal processing and are not able to offer a good control of the introduced phase shift.